I have no idea what you meant by increase muscle mass and decrease flexibility and mobility...
I do stretching on day (after workout) and off day but not so consistently. I usually stretch my weak side - wrists, forearms & elbows (had some injuries there), and just work on overall flexibility and mobility for the shoulders, wrists, forearms & elbows, legs, lower back and so on.
About the deload, I believe Chris recommends to take a full week off every 4th month of training to avoid injuries. And obviously you can implement actual deloads every 3rd or 4th week, just joke around perhaps, I personally just do some skill training, kind of as a fun deload week and I usually do it whenever I feel some tightness or injury could come so I just stop doing sets & reps and do whatever I feel like if you know what I mean.